Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River

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Table 3. Step -trend results on specific conductance at station 07096000 (Arkansas River at Canon City) between 1964 -74 and 1975-94 [µWcm, microsiemens per centimeter at 25 degrees Celsius; N, number of values; p value is the significance level of the test; <, less than; NS, trend not statistically significant; D, statistically significant decreasing trend] to statistically significant trend was defined as having a p value less than or equal to 0.05. The median annual streamflow at station 07099400 increased from about 445,200 acre -ft /yr in 1966 -74 to about 518,400 acre -ft/yr in 1975 -94; the difference in the median annual streamflow was not statistically significant (p= 0.23). Although there was no significant change in the median annual stream- flow, there were differences in the daily mean stream- flow that were segregated by month in each of the two periods (fig. 9). Following the construction of Pueblo Reservoir, the daily mean streamflow increased significantly in March, April, June, July, September, and October (table 4). Most of the increases occurred during the growing season when demands for irriga- tion water were largest. Significant decreases in streamflow occurred in January, February, May, November, and December (table 4). This basic pattern of increased spring, summer, and fall stream- flow and decreased winter streamflow is largely attrib- utable to the WWSP. As discussed in the "Pueblo Reservoir" section, the WWSP allows irrigators to store water in Pueblo Reservoir from November 15 to March 15. This stored water usually is released to the river to meet downstream irrigation needs during early spring and the latter parts of the growing season when streamflow generally is insufficient to meet irrigation needs. The net effect of this operation downstream from Pueblo Reservoir is decreased streamflow in November through February and increased streamflow during early spring and late summer or autumn. Specific conductance at station 07099400 has changed markedly since the construction of Pueblo Reservoir (fig. 10). The median specific conductance decreased significantly (p <0.01) from 625 gS /cm in 1966 -74 to 496 g6/cm in 1975 -94. The most obvious change in specific conductance was a narrowing of the range in specific conductance that occurred after 1974. Since 1974, the annual maximum specific conductance has tended to decrease, and the annual minimum specific conductance has tended to increase slightly. The annual maximum specific conductance decreased from a range of about 800 to more than 1,000 gS /cm in 1966 -74 to a range of about 500 to 750 gS /cm in 1975 -94. Specific conductance tended to decrease during low flow, September through April, and increase during high flow, May through August (fig. 11). RELATIONS OF STREAMFLOW AND SPECIFIC - CONDUCTANCE TRENDS TO RESERVOIR OPERATIONS 17 IN THE ARKANSAS RIVER 1964 -74 1975 -94 Median Median Month specific specific p value Significances conductance N conductance N (RS/cm) (µS /cm) January 312 11 255 9 0.03 D February 321 7 273 4 .02 D March 334 12 276 10 <01 D April 333 9 326 10 .33 NS May 208 8 188 14 .52 NS June 173 9 158 13 .87 NS July 161 10 208 10 .17 NS August 226 9 250 14 .06 NS September 278 8 330 6 .11 NS October 322 12 322 9 .97 NS November 311 11 280 5 .19 NS December 318 7 266 6 .06 NS to statistically significant trend was defined as having a p value less than or equal to 0.05. The median annual streamflow at station 07099400 increased from about 445,200 acre -ft /yr in 1966 -74 to about 518,400 acre -ft/yr in 1975 -94; the difference in the median annual streamflow was not statistically significant (p= 0.23). Although there was no significant change in the median annual stream- flow, there were differences in the daily mean stream- flow that were segregated by month in each of the two periods (fig. 9). Following the construction of Pueblo Reservoir, the daily mean streamflow increased significantly in March, April, June, July, September, and October (table 4). Most of the increases occurred during the growing season when demands for irriga- tion water were largest. Significant decreases in streamflow occurred in January, February, May, November, and December (table 4). This basic pattern of increased spring, summer, and fall stream- flow and decreased winter streamflow is largely attrib- utable to the WWSP. As discussed in the "Pueblo Reservoir" section, the WWSP allows irrigators to store water in Pueblo Reservoir from November 15 to March 15. This stored water usually is released to the river to meet downstream irrigation needs during early spring and the latter parts of the growing season when streamflow generally is insufficient to meet irrigation needs. The net effect of this operation downstream from Pueblo Reservoir is decreased streamflow in November through February and increased streamflow during early spring and late summer or autumn. Specific conductance at station 07099400 has changed markedly since the construction of Pueblo Reservoir (fig. 10). The median specific conductance decreased significantly (p <0.01) from 625 gS /cm in 1966 -74 to 496 g6/cm in 1975 -94. The most obvious change in specific conductance was a narrowing of the range in specific conductance that occurred after 1974. Since 1974, the annual maximum specific conductance has tended to decrease, and the annual minimum specific conductance has tended to increase slightly. The annual maximum specific conductance decreased from a range of about 800 to more than 1,000 gS /cm in 1966 -74 to a range of about 500 to 750 gS /cm in 1975 -94. Specific conductance tended to decrease during low flow, September through April, and increase during high flow, May through August (fig. 11). RELATIONS OF STREAMFLOW AND SPECIFIC - CONDUCTANCE TRENDS TO RESERVOIR OPERATIONS 17 IN THE ARKANSAS RIVER